Tank-tankless water heater

ABSTRACT

A tank-tankless water heater includes primary and secondary heat exchangers, and a combustor for the production of flue gases. In operation, water is first heated as the water and flue gases flow through primary heat exchanger. The water flows into the tank where it is stored and again heated as the flue gases flow through the secondary heat exchanger. A pump moves the water from the secondary heat exchanger, through the primary heat exchanger, and back to the secondary heat exchanger for storage as needed to maintain the stored water at a desired temperature. Water is drawn from the secondary heat exchanger during initial demand to provide a ready source of hot water, and the hot water supply is maintained by the primary heat exchanger during sustained hot water draws. The primary heat exchanger may include a temperature or temperature differential control system.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/015,190 filed on Jan. 16, 2008, the contents of which areincorporated herein by reference, and claims priority to U.S.Provisional Patent Application No. 60/902,566 filed on Feb. 21, 2007,the contents of which are incorporated herein by reference. Thisapplication also claims priority to U.S. Provisional Patent ApplicationNo. 60/972,146 filed on Sep. 13, 2007, the contents of which areincorporated herein by reference.

BACKGROUND

Generally, water heaters fall into one of two types: (i) tankless orinstantaneous water heaters, and (ii) storage or tank water heaters.Each type of water heater has its advantages and disadvantages, and thedecision to use one over the other for a particular application involvestrade-offs in various performance issues. The present invention relatesto a water heater that takes advantage of beneficial aspects of bothwater heater types while avoiding some disadvantages of each.

SUMMARY

In one embodiment, the invention provides a tank-tankless water heatercomprising: a combustor for the production of hot flue gases; a primaryheat exchanger including a core and a flue gas flow path; and asecondary heat exchanger including a tank and at least one flue. Fluegases flow from the combustor through the flue gas flow path and thenthrough the at least one flue. Water to be heated first flows throughthe core, then into the tank where the water is stored, and then flowsout of the tank for use upon demand. Heat is transferred from the fluegases to the water first as the water flows through the core and theflue gases flow through the flue gas flow path, and again as the wateris stored in the tank and the flue gases flow through the at least oneflue.

In some embodiments, the primary heat exchanger includes a primary waterinlet that delivers water to be heated to the core, and a primary wateroutlet that delivers heated water from the core to the tank. The primaryheat exchanger may be a temperature controlled heat exchanger having aflow control valve operable to selectively restrict flow of waterthrough the core to achieve a desired water temperature at the primarywater outlet. In other embodiments, the primary heat exchanger is atemperature differential controlled heat exchanger in which thetemperature of water flowing through the core from the primary waterinlet to the primary water outlet is raised a substantially fixedamount.

In some embodiments, the water heater also includes a water pumpcommunicating between the tank and the core and operable to move waterfrom the tank, through the core, and back to the tank, to heat the waterand raise the temperature of water in the tank. The pump may be operableto move water from a bottom portion of the tank, then through the core,and then to a top portion of the tank. The pump may alternatively beoperable to move water from a top portion of the tank, then through thecore, and then to a bottom portion of the tank. A temperature sensor maybe used for sensing water temperature in the tank and activating thewater pump in response to the water temperature in the tank fallingbelow a set point temperature.

In some embodiments, the water heater includes a flow activationcontroller operable to initiate operation of the combustor in responseto water flow through the core.

In some embodiments, the water heater includes a water flow circuitoperable, in response to a performance draw of hot water from the tank,to draw hot water from the tank at a first temperature, mix the hotwater with cold water to produce reduced temperature water at atemperature lower than the first temperature, flow the reducedtemperature water through the primary heat exchanger to produce reheatedwater at a second temperature substantially equal to the firsttemperature, and returning the reheated water to the tank.

In some embodiments, the primary heat exchanger includes a primary waterinlet and a primary water outlet; the secondary heat exchanger includesa secondary water inlet communicating with the primary water outlet forreceiving hot water from the primary heat exchanger, a secondary wateroutlet through which hot water flows out of the tank for use upondemand, and a two-way port; the water heater further comprises a teecommunicating between the primary water inlet and the two-way port, andadapted to communicate with a source of cold water; upon demandreplacement cold water from the source of cold water replaces hot waterdrawn from the tank; and at least some of the replacement cold waterflows through the two-way port into the tank without flowing through theprimary heat exchanger.

In some embodiments, the water heater further comprises a temperaturesensor generating a signal in response to water temperature in the tankfalling below a set point during continued flow of water out of the tankfor use; a water pump; and a controller activating the pump in responseto receiving the signal to direct an increased amount of cold water fromthe tee to the primary water inlet and thereby reduce the amount of coldwater entering the tank through the two-way port. In some embodiments,the water heater further comprises a temperature sensor generating asignal in response to water temperature in the tank falling below a setpoint during continued flow of water out of the tank for use; and acontroller restricting cold water flow through the bypass circuit inresponse to receiving the signal, to increase an amount of cold waterflowing through the primary heat exchanger prior to entering the tankafter the signal is generated. In some embodiments, the water heaterfurther comprises means for increasing the flow of cold water from thetee to the primary water inlet and decreasing the flow of cold waterfrom the tee to two-way port; wherein cold water is introduced to abottom portion of the tank through the two-way port; and wherein wateris introduced to the top portion of the tank from the primary heatexchanger.

In some embodiments, the water heater further comprises: a first sensorcoupled to a lower portion of the tank for generating a first signalindicative of water temperature within the lower portion of the tank; asecond sensor coupled to an upper portion of the tank for generating asecond signal indicative of water temperature within the upper portionof the tank; a two-way port communicating with the lower portion of thetank; a cold water supply line communicating with both the primary waterinlet and the two-way port; a proportional valve communicating betweenthe cold water supply line and the two-way port; and a water pumpcommunicating between the cold water supply line and the primary heatexchanger; wherein cold water flows into the tank through the two-wayport during initial performance draw of hot water from the tank; whereinthe water pump is energized in response to the first sensor generatingthe first signal, such that a portion of cold water from the cold watersupply line flows through the primary heat exchanger before reaching thetank; and wherein the proportional valve restricts flow of cold waterthrough the two-way valve in response to the second sensor generatingthe second signal.

In some embodiments, the water heater further comprises a flow sensormonitoring the flow of hot water during a performance draw; wherein theflow sensor causes the proportional valve to increase the flow of coldwater through the two-way valve in response to the performance drawending. In some embodiments, the pump draws water from the tank throughthe two-way valve, flows the water through the primary heat exchangerwhere the water is reheated, and returns the reheated water to the tankin the absence of a performance draw in response to at least one of thefirst and second signals being generated.

The invention also provides a method of heating water, comprising thesteps of: (a) providing a primary heat exchanger having a core and aflue gas flow path; (b) providing a secondary heat exchanger including atank and at least one flue; (c) producing hot flue gases; (d) moving theflue gases through the flue gas flow path and then through the at leastone flue; (e) flowing water to be heated first through the core, theninto the tank; (f) heating the water first in the primary heat exchangeras the water flows through the core and the flue gases flow through theflue gas flow path; and (g) after heating the water in the primary heatexchanger, storing the water in the tank and heating the water in thetank as the flue gases flow through the at least one flue.

In some embodiments, the method may also include sensing a temperatureof the water stored in the tank and moving water from the tank, throughthe core, and back to the tank to reheat the water stored in the tank inresponse to the water temperature in the tank falling below a set pointtemperature.

In some embodiments, step (f) may include selectively restricting theflow of water through the core to achieve a desired temperature of waterflowing out of the primary heat exchanger, and step (e) may includeintroducing water from the core into a top portion of the tank.

In some embodiments, step (f) may include raising the temperature ofwater flowing through the core a fixed amount, and step (e) may includeintroducing water from the core into a bottom portion of the tank. Themethod may also include the steps of (h) providing hot water from a topportion of the tank to a user; and (i) in response to step (h), movinghot water at a first temperature out of the top portion of the tank,mixing the hot water with cold water to create reduced temperaturewater, flowing the reduced temperature water through the core to createreheated water having a second temperature substantially equal to thefirst temperature, and introducing the reheated water into the bottomportion of the tank.

In some embodiments, the method may also include the following steps:(h) providing hot water from a top portion of the tank to a user; (i) inresponse to step (h), bypassing the primary heat exchanger to directcold water directly into a bottom portion of the tank to replace waterflowing out of the tank; (j) monitoring water temperature in the tank;and (k) diverting a portion of cold from flowing directly into thebottom portion of the tank, and flowing the diverted cold water throughthe primary heat exchanger and then into a top portion of the tank inresponse to water temperature in the tank being below a cut-outtemperature.

In some embodiments, step (d) includes transferring sufficient heat fromthe flue gases to the water in the secondary heat exchanger to createcondensation of water vapors in the flue gases in the at least one flue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a first embodiment of a waterheater according to the present invention.

FIG. 2 is a schematic representation of a second embodiment of a waterheater according to the present invention.

FIG. 3 is a schematic representation of a third embodiment of a waterheater according to the present invention.

FIG. 4 is a schematic representation of a fourth embodiment of a waterheater according to the present invention.

FIG. 5 is a schematic representation of a fifth embodiment of a waterheater according to the present invention.

FIG. 6 is a schematic representation of a sixth embodiment of a waterheater according to the present invention.

FIG. 7 is a schematic representation of an alternative water circuitaccording to the present invention.

FIG. 8 is a schematic representation of an alternative control systemaccording to the present invention.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

Embodiment 1

FIG. 1 is a schematic representation of a first embodiment of atank-tankless water heater 10 according to the present invention. Theterm “tank-tankless water heater,” as used herein, refers to a waterheater that includes components and functionality of both general typesof water heaters (tankless and tank water heaters). While the focus ofthe illustrated embodiments is primarily on tank-tankless water heatersfor residential applications, it is within the scope of the invention toapply the structure and functionality of the illustrated embodiments toindustrial, commercial, and other applications not specificallydisclosed herein.

It is common to the design of storage type water heaters to have a largestorage capacity and a low input rate, while by contrast tankless typewater heaters have a very small storage capacity and large input rate.The present invention uses a combination of storage capacity and inputrate to provide the hot water needs for a residential or commercialapplication covering both the dump load (large hot water draws overshort periods) and continuous flow type of hot water usage patterns. Itis envisioned that the water heater can define a relatively smaller sizeor total volume in comparison with typical storage type water heaters.It is also envisioned that the water heater may have a lower input ratein comparison with tankless type water heaters designed for the same hotwater usage application, and therefore may not require upgrades of thegas distribution and metering system or special requirements regardingventing of flue gas.

The water heater 10 includes a primary heat exchanger 15, a secondaryheat exchanger 20, a water circuit 25, a flue gas circuit 30, and acontrol system 35. The entire water heater 10 may be enclosed in a waterheater outer casing in some embodiments. Following is a detaileddescription of the water heater 10, which is then followed bydescriptions of alternative embodiments of the invention. For the sakeof brevity, it is to be understood that aspects of each embodiment maybe incorporated into the other embodiments, and vice-versa, withoutspecific reference to same in this written description. Indeed, whereelements are similar in the various embodiments, the same referencenumerals are used in the drawings, despite such elements not alwaysbeing referenced in the written description for all of the embodiments.

Primary Heat Exchanger

In the illustrated embodiment, the primary heat exchanger 15 includes atankless water heater, which may also be referred to as the “heatengine” of the water heater 10. The primary heat exchanger 15 includesan enclosure 40 defining an interior space 45, a fuel and air intake 50,a combustor or combustion system 55, a primary heat transfer core 60within the interior space 45, a primary water inlet 65, a primary wateroutlet 70, and a primary exhaust 75. The primary core 60 is adapted forthe flow of water therethrough, and is shown schematically as a singlecoil. In other embodiments, the primary core 60 may include one or morefinned tubes, coils, and/or fin-type heat exchangers.

The primary heat exchanger 15 may be of the temperature controlled type,and may include a flow control valve 77. The flow control valve 77 maybe used to slow down the flow of water through the core 60. As waterflow rate in the core 60 is reduced, residence time of water in the core60 is increased, and more heat is transferred to the water. With properoperation of the flow control valve 77, the temperature controlledprimary heat exchanger 15 may deliver water at the primary water outlet70 at a desired temperature (e.g., 140°-150° F. or higher depending onthe application) without regard to the temperature of the water flowinginto the primary water inlet 65.

The combustor 55 is illustrated within the enclosure for example, butmay be inside or outside of the enclosure 40 in other embodiments. Thecombustor 55 may include a fixed input type or a modulating input typecombustion system. If the combustor 55 includes a modulating input type,it can be used in conjunction with the flow control valve 77 to providewater at a desired temperature at the primary water outlet 70 (i.e.,both water flow rate and combustor input rate can be adjusted to achievethe desired result). The combustor or combustion system 55 may bedesigned based on low NOx principles as well as high combustion and heattransfer efficiency.

Air and fuel are drawn into the primary heat exchanger 15 via the airand fuel intake 50, to create an air/fuel stream 80. The air/fuel stream80 may be partially premixed or fully premixed. The air/fuel stream 80is combusted in the combustor 55 to produce products of combustion orflue gases 85. The interior space 45 may be divided or partitioned tocause flue gases 85 to travel across one side of the core 60, and thenback along an opposite side of the core 60 in a double-passconfiguration. Water to be heated flows into the primary core 60 throughthe primary water inlet 65. The flue gases 85 follow a flue gas flowpath through the interior space 45 over the primary core 60, and heat istransferred from the flue gases 85 to the water flowing through theprimary core 60. As heat is transferred to the water in the primary core60, the water temperature rises and the enclosure 40 and heat exchangesurfaces (e.g., fins and the like) in the primary core 60 are cooled.Proper water flow control reduces the likelihood of local boiling in theprimary core 60, which facilitates higher heat flux density in theinterior space 45. The flue gases 85 flow out of the primary exhaust 75,and the now-heated water flows out of the primary water outlet 70.

Secondary Heat Exchanger

The secondary heat exchanger 20 includes a tank-type water heater havinga tank 90, one or more flues 95 within the tank 90, optional baffles 97in the flues 95, a flue gas inlet 100, an optional plenum 103, asecondary exhaust 105, a secondary water inlet 110, a secondary wateroutlet 115, and a two-way port 120. The flue gases 85 flow through theflue gas inlet 100, into the plenum 103, through the flues 95, and outthe secondary exhaust 105 to the atmosphere. The plenum 103 evenlydistributes the flue gases 85 into the flues 95. The baffles 97 increasedwell time of the flue gases 85 in the secondary heat exchanger 20 andenhance the heat transfer to water through the flue walls. The baffles97 can be embedded in the flue walls, or placed inside the flue 95passageway with no permanent contact to the flue walls.

Water flows into the tank 90 through the secondary water inlet 110, andis heated by heat transfer from the flue gases 85 through the fluewalls. Upon demand during a performance draw, the water in the tank 90flows out through the secondary water outlet 115, is selectively mixedwith cold water at a mixing valve 125 to achieve the desiredtemperature, and is delivered to a user at a hot water outlet or faucet127. The tank thermostat set point temperature may be higher than themixing valve set-point temperature (e.g. by about 10° F.) and also thetankless set-point (for a temperature controlled tankless heatexchanger) may be higher than the tank thermostat set point (e.g. byabout 10° F.).

Water Circuit

The water circuit 25 includes a circulating pump 130, the tank 90, thetwo-way port 120, a tee 135, the primary water inlet 65, the primarycore 60, the primary water outlet 70, and the secondary water inlet 110.When activated, the circulating pump 130 draws water from the tank 90(e.g., from the bottom of the tank in the illustrated embodiment)through the two-way port 120 and tee 135, and introduces it into theprimary heat exchanger 15 through the primary water inlet 65. Heat istransferred to the water as it flows through the primary heat exchanger15 in the primary core 60. The water, still moving under the influenceof the pump 130, flows out of the primary heat exchanger 15 through theprimary water outlet 70, and returns to the top of the tank 90 (throughthe secondary water inlet 110).

Flue Gas Circuit

The flue gas circuit 30 includes the interior space 45 around theprimary core 60, the primary exhaust 75, a flue gas circulation tube140, the flue gas inlet 100, the plenum 103, the flues 95, and thesecondary exhaust 105. Air for the air/fuel stream 80 comes from theatmosphere surrounding the primary heat exchanger 15. In someembodiments the air may be provided at higher-than-atmospheric pressureor the flue gases 85 may be flow-assisted by a fan, blower, compressoror other air moving device 145 communicating with the flue gas circuit30, upstream of the air and fuel intake 50 (as illustrated), or at thesecondary exhaust 105. In some embodiments, the primary heat exchanger15 may include its own dedicated fan, but fans in most known tanklesswater heaters may be insufficiently sized to push flue gases through theentire water heater system 10 contemplated by the present invention. Theair moving device 145, whether at the air and fuel intake 50, thesecondary exhaust 105, or somewhere in between in the flue gas circuit30, may be used to assist and supplement any dedicated fan in theprimary heat exchanger 15.

The fuel may, for example, be natural gas, propane, or anothercombustible substance, and is supplied by a source of fuel 150. Theair/fuel stream 80 is combusted to form the flue gases 85, which flowthrough the primary heat exchanger 15 as discussed above. Upon exitingthe primary heat exchanger 15 through the primary exhaust 75, thestill-hot flue gases 85 flow into the flue gas inlet 100 through theflue gas circulation tube 140. As they flow through the flues 95, theflue gases 85 transfer heat to the water in the tank 90 as discussedabove, and are exhausted to the atmosphere through the secondary exhaust105. The secondary exhaust 105 may include a chamber 155 under the tank90 and an exhaust stack 160.

The embodiment illustrated in FIG. 1 has the flue gas inlet 100 at thetop of the secondary heat exchanger 20, multiple flues 95, and thesecondary exhaust 105 at the bottom of the tank 90, but otherconfigurations of the flue gas inlet 100, flue or flues 95, andsecondary exhaust 105 are within the scope of the invention. In otherembodiments, the tank 90 and flues 95 may be turned sideways such thattheir longitudinal extents are substantially horizontal. Also, while theflues 95 illustrated in FIG. 1 are internal to the water tank 90, it ispossible to utilize a space around the outside of the tank 90 as theflue or flues 95, such that the flue gases 85 heat water in the tank 90through the tank wall. Whether the flues 95 are internal or external,they are deemed “associated with the tank” for the purposes of thiswritten description and the appended claims.

Depending on its design, the secondary heat exchanger 20 can reduce theflue gas 85 temperature down to or under the dew point of water vaporscontained in the flue gas 85. This would recover the latent heat ofcondensation of the water vapors, which may give rise to a relativelyhigher overall thermal efficiency of the water heater 10, and mayqualify the water heater 10 as a high efficiency water heater. Toaccommodate condensation, the flue surfaces over which the flue gases 85flow may be protected against water corrosion by means of one or moreprotective coatings (e.g. glass lining). If the flue gases 85 aresufficiently cool at the secondary exhaust 105, the stack 160 may beconstructed of a low-temperature and relatively inexpensive materialsuch as PVC. Also, the exhaust structure 105 may include a condensatedrain trap to collect condensed water in the secondary heat exchangerflues 95. The secondary exhaust 105 (and particularly the stack 160portion) at least partially defines the lowest temperature zone in thewater heater 10.

Control System

The control system 35 includes a thermostat/controller 165 that monitorsthe water temperature within the tank 90. The thermostat/controller 165may include a temperature probe extending into the water in the tank 90.In some embodiments, a thermostat or other temperature sensor may beprovided in each of the top (or “upper”) and bottom (or “lower”)portions of the tank 90 to generate signals related to the watertemperature in the upper and lower portions of the tank 90,respectively. The thermostat 165 activates the pump 130 when watertemperature within the tank 90 drops below a set point. The combustor 55may be activated directly by the thermostat 165, or by a flow sensor inthe core 60 or another portion of the water circuit 25 such that thecombustor 55 activates in response to water flowing through the primarycore 60 under the influence of the pump 130. In some embodiments, thecontroller 165 may control the combustor 55 (e.g., if the combustor 55is an input modulation combustor), the flow control valve 77, and anyblowers, fans, or other air-moving device 145 communicating with theflue gas circuit 30, or a separate controller may be provided for thosefunctions.

In some embodiments, the water heater 10 can include a flow sensor orflow switch upstream of the mixing valve 125 to monitor the state of thehot water draw. When the draw ends, a controller can activate the pump130 (i.e., activate the water circuit 25). As a result, water canrecirculate from the storage tank 90 through the primary heat exchanger15 and back to the storage tank 90 until the water temperature in thestorage tank 90 has recovered a desired temperature after a performancedraw.

Operation

There are two basic modes of operation for the water heater: standbymode (which also includes initial start-up, when the entire system isoriginally filled with cold water) and performance draw mode. In bothmodes, a call for heat is generated by the thermostat/controller 165 inresponse to sensing a drop in water temperature in the tank 90 below afirst limit temperature, and the pump 130 activates in response toreceiving the call for heat from the thermostat/controller 165.

In performance draw mode, hot water is delivered to the fixture 127 fromthe storage tank 90. Cold water flows into the tank 90 through thetwo-way port 120 from the tee 135 to replace water being drawn from thetank 90. As the performance draw continues, more cold water enters thebottom of the tank 90, and the water temperature in the tank 90decreases. If the water temperature in the tank 90 drops below the firstlimit temperature, the call for heat is generated and the pump 130 isactivated.

Once the pump 130 is activated, the cold water at the tee 135 followsthe path of least hydraulic resistance, either directly into the bottomof the tank 90 through the two-way port 120 or through the primary heatexchanger 15. The split in-between the two streams is done automaticallybased on the hydraulic resistance of both water paths. The flow sensorembedded into the heat engine 15 detects the flow from the pump 130 andstarts the combustion system 55; as a result the primary heat exchanger15 will start generating hot water and returning it to the storage tank90 through the secondary water inlet 110. In this regard, starting thepump 130 is equivalent to starting operation of the primary heatexchanger 15 because the combustor 55 is flow-activated. The tank 90acts as a buffer between the end user and the primary heat exchanger 15.Thus, cold or partially heated water (e.g., cold sandwiches or initialcold water flow prior to the combustor 55 starting) flowing from theprimary heat exchanger 15 into the secondary heat exchanger 20 mixeswith hot water in the tank 90 prior to flowing out through the secondarywater outlet 115.

While the combustion system 55 is in operation, the flue gases 85leaving the heat engine 15 are still hot (e.g., 350° F.) and their heatwill be recovered by passing them through the secondary heat exchangerflue path 95. In order to extract the latent heat of condensation fromthe water vapor contained in the flue gas 85 (and boost the overallefficiency of the system), the flue stream 85 needs to leave the storagetank 90 through its lower portion (where water stored in the tank 90will be colder as a result of the natural tank temperaturestratification). The flue tube 95 wall in that lower tank area needs tohave a temperature below the dew point of the flue gas 85 containedwater vapors in order to promote condensation.

A temperature monitor in the primary heat exchanger 15 provides feedbackto the combustor 55 as to the temperature of water at the primary wateroutlet 70. If temperature at the primary water outlet 70 is below atarget temperature, the combustor's input rate is increased (if it is amodulated unit). If the primary heat exchanger 15 requires an input ratethat is larger than the maximum input rate of the combustor 55, then thewater flow control valve 77 will start to restrict the flow through thecore 60. The flow control valve 77 increasingly restricts flow until thetarget temperature is achieved at the primary water outlet 70. As theflow control valve 77 restricts flow, the water flow rate circulated bythe pump 130 will be lower than the maximum one allowed by the hydraulicresistance of the system.

Cold water entering the water heater 10 will naturally follow the pathof least hydraulic resistance, and thus some cold water will likely flowinto the tank 90 through the two-way port 120 even when the pump 130 isrunning As the hydraulic resistance through the primary heat exchanger15 increases, however, the amount of cold water flowing into the tank 90through the two-way port 120 increases as a percentage of total coldwater flowing into the water heater 10. Unless the demand for hot waterat the faucet 127 is decreased, the water heater 10 will eventually runout of hot water, and the performance draw will need to be stopped topermit the water heater to recover. The water heater 10 recovers byrunning the pump 130 following a performance draw, such that water andflue gases cycle through the primary heat exchanger 15 and secondaryheat exchanger 20.

The end of the call for heat occurs when the monitored temperature inthe storage tank 90 exceeds a second limit temperature, which is greaterthan the first limit temperature by a selected differential (e.g. 10°F.). The pump 130 is deactivated in response to the end of the call forheat, which in turn deactivates the combustion system 55 of the heatengine 15. The heat engine 15 will not operate if the pump 130 does notoperate.

During standby mode, the heat engine 15 is used to recharge the storagetank 90 with hot water. When the system enters this heating mode, thepump 130 draws water from the storage tank 90 through the two-way port120, circulates the water through the heat engine 15, and returns it atthe secondary water inlet 110. In standby mode, the heat engine 15operates at the maximum flow rate (i.e., the flow control valve 77 doesnot restrict the flow), allowed by the hydraulic resistance of the heatengine and connecting pipes.

In view of the above, the two-way port 120 serves two purposes in thewater circuit 25. During initial performance draw, before the pump 130is activated, substantially all hot water leaving the tank 90 isreplaced with cold water through the two-way port 120. Cold water alsocontinues to flow into the tank 90 if the pump 130 is not keeping upwith the demand for hot water. Because the cold water flows directlyinto the tank 90 through the two-way port 120 (and does not have to flowthrough the primary heat exchanger 15) under such circumstances, theport 120 acts as a bypass circuit with respect to the primary heatexchanger 15. During standby, when the tank is being recharged with hotwater, the pump 130 draws cold water out of the tank through the port120, and in this regard the port acts as a recirculation water outlet.

Water heaters according to the present invention may include improvedthermal efficiency over known tank and tankless water heaters. Morespecifically, the water heater can operate with an efficiency of about90% or more. The water heater can also replace current water heatersincluding power vent, conventional vent, and direct vent water heaters.The water heater can also include relatively short recovery times incomparison to standard storage tank water heaters. Some features of thewater heater include continuous hot water delivery for reasonable flowrates (e.g. 2.5 GPM). Another feature is the incorporation ofintelligent controls that allow an optimized use of the water heatereither directly for hot water domestic applications or as a heat sourcefor use in combination applications (e.g. convective or radiant spaceheating and hot water delivery). The water heater is envisioned ashaving various advantages over standard tank-type water heaters, such asa larger first hour rating (the amount of hot water that can bedelivered in one hour), and defining a smaller size or storage capacity.

The water heater is also envisioned as having various advantages incomparison to standard tankless type water heaters. For example, some ofthe advantages include eliminating hot water temperature spikes, whichare generally common in tankless type water heaters. This measure canreduce scalding hazards associated with tankless water heaters. Anotheradvantage of the water heater is the water heater not being limited to amaximum flow rate. The water heater according to the present inventionis capable of accommodating dump loads. Other advantages include betterinitial performance for low incoming cold water temperature, due to asmall storage buffer, and increasing the lifetime of the tankless waterheater component by using stored hot water for consumption patternsinvolving short draws. Another advantage includes relatively lowerinstallation costs by using PVC for the venting system.

The inventive features of the water heaters described in thisapplication allow the described water heaters to differ from previousstorage-tank water heater designs through the use of a compact primaryheat exchanger with controlled water circulation and high intensity(heat rate/volume) combustion system, having the tank-type component ofthe system to act as both a condensing heat exchanger and a buffer tank.Additionally, previous condensing tankless type water heaters generallyhave a secondary heat exchanger of a tankless type (coil type orfin-type). Thus, these previous tankless type water heaters differ fromthe water heaters described herein because the tank-tankless waterheaters comprise a heat exchanger acting as a storage buffer tank and assecondary heat exchanger.

Other features of the water heaters in this application are that thetankless water heater can deliver water at controlled temperatures orcontrol the temperature rise of the water. In other words, the tanklessheat exchanger can control the differential between incoming cold waterand the hot water delivered by means of fuel/air ratio and/or water flowrate modulation. The tankless water heater can act as a heating sourcetransforming the chemical energy from the fuel in heat and also asprimary heat exchanger. The primary heat exchanger can be a fin tubetype heat exchanger, in which water flows through tubes and flue gasflows over the fins on the outside the tubes. Such a heat exchanger isable to transfer large amounts of heat from the flue gas to the waterflowing through the primary heat exchanger.

A water heater according to the present invention may be modular(tankless water heaters of different inputs may be combined with storagetanks of different capacities to accommodate various hot waterapplication). Also envisioned is the use of multiple tankless waterheaters in parallel connected to a single storage tank or a singletankless water heater connected to multiple storage tanks in parallel.

Other Illustrated Embodiments

FIGS. 2, 3, 4, 5, and 6 illustrate respective second, third, fourth,fifth, and sixth embodiments of the invention. These embodiments employmuch of the same structure and have many of the same properties as theembodiment of the invention described above in connection with FIG. 1.Where similar or identical features to the first embodiment areemployed, the same reference numerals appear in the drawings. Thefollowing description focuses primarily upon the structure andfunctionality in these embodiments that are different from the firstembodiment. It should be noted that elements of any embodiment disclosedherein may in appropriate circumstances be applied to or used withinother embodiments.

FIG. 2 illustrates a water heater 210 having a secondary heat exchanger20 with a single flue 95 and the secondary exhaust 105 in a side of thetank 90, but is otherwise set up in a substantially similar manner asthe water heater 10 of the first embodiment.

FIG. 3 illustrates a water heater 310 in which the primary heatexchanger 15 is at least partially within the water tank 90. In theillustrated embodiment, all but the bottom of the heat exchangerenclosure 40 is covered with water in the tank 90. In other embodiments,more or less of the enclosure 40 may be submerged within the tank thanis illustrated schematically in FIG. 3. The secondary water inlet 110 isillustrated as being at the top of the primary heat exchanger 15, butnot at the top of the tank 90. A dip tube can be used to deliver thewater to the top of the tank 90.

The flue gas circulation tube 140 in this third embodiment includes avertical rise from the submerged primary heat exchanger enclosure 40 upthrough the water in the tank 90 to the plenum 103. In the plenum 103,the flue gases 85 turn down into the flues 95 of the secondary heatexchanger 20. The vertical rise of the flue gas circulation tube 140provides some heat transfer from flue gases 85 to the water in the tank90, and in that regard may be deemed one of the flues 95. The verticalrise 140 may be centered within the tank 90 as illustrated, or may beoff-center in other embodiments. The air moving device 145 in thisembodiment includes a blower to assist the flow of flue gases 85 upthrough the vertical rise and back down through the flues 95. Thecombustor 55 and blower 145 in this embodiment may be within the chamber155 under the tank 90.

FIG. 4 illustrates a water heater 410 in which the primary heatexchanger 15 is at least partially submerged at the top of the watertank 90. As illustrated, the secondary water inlet 110 is generally inthe middle portion of the tank 90 with this construction. The blower 145in this embodiment forces the flue gases 85 down through the single flue95 in the secondary heat exchanger 20. The combustor 55 in thisembodiment may be above the tank 90. Because the flue 95 communicatesdirectly with the interior space 45 of the enclosure 40 in thisembodiment, there is no flue gas circulation tube 140.

FIG. 5 illustrates a water heater 510 similar in all respects to theembodiment 310 illustrated in FIG. 3, except that the primary heatexchanger 15 is not submerged, but is within the chamber 155 under thetank 90. Also, in this embodiment, the secondary water inlet 110 may bein the top portion of the tank 90.

FIG. 6 illustrates a water heater 610 similar in all respects to theembodiment 410 illustrated in FIG. 4, except that the primary heatexchanger 15 is not submerged, but is above the tank 90. Also, in thisembodiment, the secondary water inlet 110 may be in the top portion ofthe tank 90.

Alternative Water Circuit

FIG. 7 illustrates a water heater 710 embodying the present inventionand including a first alternative water circuit 25′ for use with anon-temperature controlled primary heat exchanger 15. A non-temperaturecontrolled primary heat exchanger raises the temperature of water by asubstantially fixed amount for each pass through the core 60, and maythus be referred to as a temperature differential controlled heatexchanger. Thus, the temperature of water flowing out of the primarywater outlet 70 will be warmer than it was when it flowed into theprimary water inlet 65 by a substantially fixed amount. Stated anotherway, the temperature of water flowing out of the primary water outlet 70is a function of or dependent on the temperature of the water when itflowed into the primary water outlet 65 in a non-temperature controlledprimary heat exchanger 15. In one example, the primary heat exchanger 15may raise the temperature of water 40°-50° F. as it flows through thecore 60 from the primary water inlet 65 to the primary water outlet 70.This is a relatively small temperature increase when compared to atemperature controlled primary heat exchanger, such as those describedabove with respect to other embodiments.

Because the primary heat exchanger 15 raises the temperature of waterflowing through it by only a relatively small amount, water must becycled through the primary heat exchanger 15 multiple times to raise thetemperature of water in the tank 90 to a desired temperature. Each cycleadds a substantially fixed temperature rise to the water, and eventuallythe water in the tank 90 is at a temperature suitable for use (e.g.,140°-150° F. or higher for some applications).

The water circuit 25′ provides a substantially uniform water temperaturethroughout the tank 90, which maximizes hot water in the tank 90. Morespecifically, in the water circuit 25′, the secondary water inlet 110communicates with the bottom of the tank 90 and the two-way port 120communicates with the top of the tank 90. Thus, the water circuit 25′draws hot water from the top of the tank 90, raises the watertemperature as it flows through the core 60, and returns the water tothe bottom of the tank 90. The hot water delivered at the bottom of thetank 90 rises toward the top of the tank 90 by means of buoyancy andhelps ensure the mixing process.

During a performance draw, hot water is drawn from the tank 90, mixedwith cold water at the mixing valve 125, and delivered to the user atthe hot water outlet or faucet 127 as discussed above. In thisembodiment, however, the pump 130 is activated upon initiation of aperformance draw, and hot water is simultaneously drawn from the top ofthe tank 90 through the two-way outlet 120. The hot water flows from thetwo-way port 120 through the tee 135 where it is mixed with cold water,such that the hot/cold mixture flows into the primary heat exchanger 15at a reduced temperature (i.e., reduced temperature water at atemperature that is lower in temperature than the hot water by a fixedamount). The reduced temperature water then flows through the primaryheat exchanger 15, where its temperature is raised by the fixed amountto produce reheated water (i.e., water that has been heated tosubstantially the same temperature as the hot water drawn off the tank),and is returned to the bottom of the tank 90.

In one example, if the non-temperature controlled primary heat exchanger15 raises water about 40° F. (i.e., this is the “fixed amount” referredto above), and if water at the top of the tank 90 (i.e., the “hot water”referred to above) is at a temperature of about 140° F., then cold waterintroduced at the tee 135 should lower the water temperature by about40° F. to about 100° F. (i.e., the “reduced temperature water” referredto above), so that the primary heat exchanger 15 can subsequently raisethe water temperature back to 140° F. (i.e., create the “reheated water”referred to above), such that the temperature of water returning to thetank 90 is at the desired temperature of 140° F. It may be desirable insome applications to provide the reduced temperature water at atemperature that is lower in temperature than the hot water by less thanthe fixed amount (i.e., provide reduced temperature water at higher than100° in the example give), such that reheated water leaving the primaryheat exchanger 15 is above the temperature of the hot water drawn offthe top of the tank 90 (i.e., the reheated water is at a temperature inexcess of 140° F.) to offset the cooling effect of mixing the reheatedwater with potentially cooler water at the bottom of the tank 90.

During standby, the pump 130 is activated when water in the tank 90cools below a set point. The combustor in the primary heat exchanger 15may be flow activated such that it automatically starts in response towater flow through the core 60. The pump 130 continues to operate untilthe water in the tank 90 has reached a desired temperature; this mayrequire one or more cycles of water flowing through the primary heatexchanger and back to the bottom of the tank 90.

One advantage of the water circuit 25′ is that it provides asubstantially constant flow of water into the tank 90 because it doesnot use a flow restricting valve in the primary heat exchanger 15. Thus,the pump 130 can be smaller and use less power than in otherembodiments. One disadvantage of the alternative water circuit 25′ isthat it less accurately controls the temperature of water than otherembodiments using temperature controlled primary heat exchangers. Thus,the mixing valve 125 may need to accommodate wider fluctuations in watertemperature from the tank 90 to accurately control water temperatures atthe hot water outlet 127. The water heater 710 also requires a largercapacity tank 90 in the secondary water heater 20 to accommodatetemperature fluctuations at the secondary water inlet 110 arising from aless accurate primary heat exchanger.

This embodiment and all other embodiments described may includeadditional elements, such as a pressure regulator 720 to controlpressure of water from a cold water source, and expansion tank 730, anda temperature and pressure (T&P) relief valve 740 coupled to the tank90.

Alternative Control System

FIG. 8 illustrates a water heater 810 embodying the present inventionand including an alternative control system 35′. The water heater 810includes an outer casing 815 enclosing the primary heat exchanger 15 andthe secondary heat exchanger 20 (as stated above, a similar casing maybe applied to any previously-described embodiment as well). Thealternative control system 35′ includes a first temperature sensor 820mounted in a lower portion of the tank 90, a second temperature sensor825 mounted in an upper portion of the tank 90, a controller 830, aproportional valve 835, a flow sensor 840, and a high limit switch 845.

During a performance draw, hot water is initially drawn from the top ofthe storage tank 90 of the secondary heat exchanger 20. Hot water fromthe storage tank 90 is selectively mixed with cold water in the mixingvalve 125 to achieve a requested temperature at the hot water outlet127. The flow of water out of the water heater 810 to the faucet 127 ismonitored by the flow sensor 840.

As hot water is initially drawn out of the storage tank 90, theproportional valve 835 is wide open. Cold water follows the path ofleast resistance at the tee 135 and flows directly into the bottom ofthe tank 90 through the two-way port 120. Consequently, water drawn fromthe tank 90 is replaced with cold water introduced into the bottom ofthe storage tank 90. When the first temperature sensor 820 senses thatthe water temperature at the bottom of the tank 90 has fallen below afirst temperature limit, the first temperature sensor 820 generates afirst signal to the controller 830. In response to receiving the firstsignal, the controller 830 activates the pump 130, such that cold wateris directed from the tee 135 through the primary heat exchanger 15 andinto the top of the tank 90. The primary heat exchanger 15 istemperature controlled, and restricts flow of cold water with the flowrestrictor 77 when the combustor 55 is unable to meet the input raterequired of the primary heat exchanger. The controller 830 may alsocontrol the flow control valve 77, or in other embodiments, the flowcontrol valve 77 may be controlled by a separate controller in theprimary heat exchanger 15. In a long, sustained performance draw, hotwater in the tank 90 is eventually depleted if the primary heatexchanger 15 cannot keep up with the demand at the outlet 127, becausecold water flowing into the tank 90 via the two-way port 120 exceeds hotwater flowing into the tank 90 from the primary heat exchanger 15.

To this point, the water heater 810 operates in substantially identicalfashion to the water heater 10 of the first embodiment. This embodimentof the water heater 810 differs from the first embodiment 10, however,in how it reacts to hot water depletion. In the first embodiment, theuser was obligated to stop the performance draw by turning off thefaucet 127, and wait for the water heater 10 to recover. In thisembodiment 810, when the second temperature sensor 825 senses that watertemperature at the top of the tank 90 has dropped below a secondtemperature limit indicative of hot water depletion, the secondtemperature sensor 825 generates a second signal to the controller 830.In response to receiving the second signal, the controller 830 actuatesthe proportional valve 835 to restrict cold water flow into the bottomof the tank 90 through the two-way port 120.

As the hydraulic resistance is increased in the proportional valve 835,the flow rate of hot water out of the tank 90 may exceed the supply ofhot water from the primary heat exchanger 15, in which case more coldwater is delivered into the tank 90 through the two-way port 120. Thehot water supplied by the primary heat exchanger 15 flows substantiallydirectly through the storage tank 90 (across the top portion of the tank90) to the secondary water outlet 115 connected to mixing valve 125. Theresult of restricting flow into the tank 90 through the two-way port 120and forcing most or substantially all cold water to flow through theprimary heat exchanger 15 is that the flow rate of hot water supply atthe faucet 127 will be substantially limited to the flow rate permittedby the flow restrictor 77. One advantage that this alternative controlsystem 35′ has over the control system 35 of previous embodiments isthat the water heater 810 will provide an “endless” supply of hot water,although the flow rate of such hot water may be restricted (i.e., asrequired by the primary heat exchanger 15 to achieve sufficiently hightemperatures) after the tank 90 is depleted.

When the draw ends, the flow sensor 840 generates a recharge signal tothe controller 830. In response to receiving the recharge signal, thecontroller 830 opens the proportional valve 835, and if the watertemperature in the tank 90 requires reheating, activates the pump 130(or continues to operate the pump 130 if it was already activated duringthe just-ended performance draw). The pump 130 recirculates the waterfrom two-way port 120 of the tank 90, through the primary heat exchanger15, and back to the tank 90 through the secondary water inlet 110 untilthe water temperature in the storage tank 90 has recovered a desiredtemperature (which may be set above the first and/or second temperaturelimits).

The controller 830 also communicates with the high limit switch 845. Thehigh limit switch 830 is in or upstream of the flue gas exhaust 105. Inthis embodiment 810, the air moving device 145 may take the form of anexhaust fan. The high limit switch 830 detects the temperature of theflue gas 85 flowing between the fan 145 and the flue gas exhaust 105,and shuts down the water heater 810 if the flue gas temperature exceedsthe temperature for which the exhaust duct 160 material, fan 145, orother component is rated.

In this embodiment 810, the flue gas circulation tube 140 connects theprimary heat exchanger 15 to the lower portion of the secondary heatexchanger 20, and the flue gas flows from the lower portion to the upperportion of the secondary heat exchanger 20. A connection tube 850communicates between the secondary heat exchanger 20 and the exhaust fan145. Condensate is permitted to drip out of the connection tube 850 andthe fan 145 (via conduit 855) into a condensate drain trap 860.

Various features and advantages of the invention are set forth in thefollowing claims.

What is claimed is:
 1. A tank-tankless water heater comprising: acombustor for the production of hot flue gases; a primary heat exchangerincluding a core and a flue gas flow path; and a secondary heatexchanger including a tank and at least one flue; wherein flue gasesflow from the combustor through the flue gas flow path and then throughthe at least one flue; wherein water to be heated first flows throughthe core, then into the tank where the water is stored, and then flowsout of the tank for use upon demand; and wherein heat is transferredfrom the flue gases to the water first as the water flows through thecore and the flue gases flow through the flue gas flow path, and againas the water is stored in the tank and the flue gases flow through theat least one flue.
 2. The water heater of claim 1, wherein the primaryheat exchanger includes a primary water inlet that delivers water to beheated to the core, and a primary water outlet that delivers heatedwater from the core to the tank; and wherein the primary heat exchangeris a temperature controlled heat exchanger having a flow control valveoperable to selectively restrict flow of water through the core toachieve a desired water temperature at the primary water outlet.
 3. Thewater heater of claim 1, wherein the primary heat exchanger includes aprimary water inlet that delivers water to be heated to the core, and aprimary water outlet that delivers heated water from the core to thetank; and wherein the primary heat exchanger is a temperaturedifferential controlled heat exchanger in which the temperature of waterflowing through the core from the primary water inlet to the primarywater outlet is raised a substantially fixed amount.
 4. The water heaterof claim 1, further comprising a water pump communicating between thetank and the core and operable to move water from the tank, through thecore, and back to the tank, to heat the water and raise the temperatureof water in the tank.
 5. The water heater of claim 4, wherein the waterpump is operable to move water from a bottom portion of the tank, thenthrough the core, and then to a top portion of the tank.
 6. The waterheater of claim 4, wherein the water pump is operable to move water froma top portion of the tank, then through the core, and then to a bottomportion of the tank.
 7. The water heater of claim 4, further comprisinga temperature sensor sensing water temperature in the tank, thetemperature sensor activating the water pump in response to the watertemperature in the tank falling below a set point temperature.
 8. Thewater heater of claim 1, further comprising a flow activation controlleroperable to initiate operation of the combustor in response to waterflow through the core.
 9. The water heater of claim 1, furthercomprising a water flow circuit operable, in response to a performancedraw of hot water from the tank, to draw hot water from the tank at afirst temperature, mix the hot water with cold water to produce reducedtemperature water at a temperature lower than the first temperature,flow the reduced temperature water through the primary heat exchanger toproduce reheated water at a second temperature substantially equal tothe first temperature, and returning the reheated water to the tank. 10.The water heater of claim 1, wherein the primary heat exchanger includesa primary water inlet and a primary water outlet; wherein the secondaryheat exchanger includes a secondary water inlet communicating with theprimary water outlet for receiving hot water from the primary heatexchanger, a secondary water outlet through which hot water flows out ofthe tank for use upon demand, and a two-way port; the water heaterfurther comprising a tee communicating between the primary water inletand the two-way port, and adapted to communicate with a source of coldwater; wherein upon demand replacement cold water from the source ofcold water replaces hot water drawn from the tank; and wherein at leastsome of the replacement cold water flows through the two-way port intothe tank without flowing through the primary heat exchanger.
 11. Thewater heater of claim 10, further comprising a temperature sensorgenerating a signal in response to water temperature in the tank fallingbelow a set point during continued flow of water out of the tank foruse; a water pump; and a controller activating the pump in response toreceiving the signal to direct an increased amount of cold water fromthe tee to the primary water inlet and thereby reduce the amount of coldwater entering the tank through the two-way port.
 12. The water heaterof claim 10, further comprising a temperature sensor generating a signalin response to water temperature in the tank falling below a set pointduring continued flow of water out of the tank for use; and a controllerrestricting cold water flow through the two-way port into the tank inresponse to receiving the signal, to increase an amount of cold waterflowing through the primary heat exchanger prior to entering the tankafter the signal is generated.
 13. The water heater of claim 10, furthercomprising means for increasing the flow of cold water from the tee tothe primary water inlet and decreasing the flow of cold water from thetee to the two-way port; wherein cold water is introduced to a bottomportion of the tank through the two-way port; and wherein water isintroduced to a top portion of the tank from the primary heat exchanger.14. The water heater of claim 1, further comprising: a first sensorcoupled to a lower portion of the tank for generating a first signalindicative of water temperature within the lower portion of the tank; asecond sensor coupled to an upper portion of the tank for generating asecond signal indicative of water temperature within the upper portionof the tank; a two-way port communicating with the lower portion of thetank; a cold water supply line communicating with both the primary waterinlet and the two-way port; a proportional valve communicating betweenthe cold water supply line and the two-way port; and a water pumpcommunicating between the cold water supply line and the primary heatexchanger; wherein cold water flows into the tank through the two-wayport during initial performance draw of hot water from the tank; whereinthe water pump is energized in response to the first sensor generatingthe first signal, such that a portion of cold water from the cold watersupply line flows through the primary heat exchanger before reaching thetank; and wherein the proportional valve restricts flow of cold waterthrough the two-way port in response to the second sensor generating thesecond signal.
 15. The water heater of claim 14, further comprising aflow sensor monitoring the flow of hot water during a performance draw;wherein the flow sensor causes the proportional valve to increase theflow of cold water through the two-way valve in response to theperformance draw ending.
 16. The water heater of claim 14, wherein thepump draws water from the tank through the two-way port, flows the waterthrough the primary heat exchanger where the water is reheated, andreturns the reheated water to the tank in the absence of a performancedraw in response to at least one of the first and second signals beinggenerated.
 17. The water heater of claim 1, wherein the at least oneflue extends between a top portion and bottom portion of the tank; andwherein flue gases flow up through the at least one flue from the bottomportion to the top portion of the tank.
 18. The water heater of claim 1,wherein the at least one flue extends between a top portion and bottomportion of the tank; and wherein flue gases flow down through the atleast one flue from the top portion to the bottom portion of the tank.19. The water heater of claim 1, wherein the primary heat exchanger issubstantially entirely within the tank of the secondary heat exchanger.20. A method of heating water, comprising the steps of: (a) providing aprimary heat exchanger having a core and a flue gas flow path; (b)providing a secondary heat exchanger including a tank and at least oneflue; (c) producing hot flue gases; (d) moving the flue gases throughthe flue gas flow path and then through the at least one flue; (e)flowing water to be heated first through the core, then into the tank;(f) heating the water first in the primary heat exchanger as the waterflows through the core and the flue gases flow through the flue gas flowpath; and (g) after heating the water in the primary heat exchanger,storing the water in the tank and heating the water in the tank as theflue gases flow through the at least one flue.
 21. The method of claim20, further comprising sensing a temperature of the water stored in thetank and moving water from the tank, through the core, and back to thetank to reheat the water stored in the tank in response to the watertemperature in the tank falling below a set point temperature.
 22. Themethod of claim 20, wherein step (f) includes selectively restrictingthe flow of water through the core to achieve a desired temperature ofwater flowing out of the primary heat exchanger.
 23. The method of claim22, wherein step (e) includes introducing water from the core into a topportion of the tank.
 24. The method of claim 20, wherein step (f)includes raising the temperature of water flowing through the core afixed amount.
 25. The method of claim 24, wherein step (e) includesintroducing water from the core into a bottom portion of the tank. 26.The method of claim 24, further comprising the steps of (h) providinghot water from a top portion of the tank to a user; and (i) in responseto step (h), moving hot water at a first temperature out of the topportion of the tank, mixing the hot water with cold water to createreduced temperature water, flowing the reduced temperature water throughthe core to create reheated water having a second temperaturesubstantially equal to the first temperature, and introducing thereheated water into the bottom portion of the tank.
 27. The method ofclaim 20, further comprising (h) providing hot water from a top portionof the tank to a user; (i) in response to step (h), bypassing theprimary heat exchanger to direct cold water directly into a bottomportion of the tank to replace water flowing out of the tank; (j)monitoring water temperature in the tank; and (k) diverting a portion ofthe cold water from flowing directly into the bottom portion of thetank, and flowing the diverted cold water through the primary heatexchanger and then into a top portion of the tank in response to watertemperature in the tank being below a cut-out temperature.
 28. Themethod of claim 20, wherein step (d) includes transferring sufficientheat from the flue gases to the water in the secondary heat exchanger tocreate condensation of water vapors in the flue gases in the at leastone flue.